ABSTRACT Women with dense breasts on mammograpm have a 4-6-fold increased risk of breast cancer. A sizeable proportion of premenopausal breast cancer cases (39%) are attributable to having dense breasts. Observational and clinical trial data have shown that a decrease in breast density translates to a reduction in breast cancer incidence. Hence, interventions to reduce breast density could prevent breast cancer. However, adult dietary and lifestyle modifications have not been shown to reduce mammographic density. Therefore, identifying a pathway that can be targeted to reduce breast density and breast cancer incidence is crucial. The receptor activator of nuclear factor-?B (RANK) pathway regulates the development of the lobulo-alveolar mammary structures, activates downstream signaling cascades involved in breast cancer and is the major mediator of progesterone-driven expansion of mammary stem cells. The RANK pathway is associated with mammographic density and breast cancer risk. This has led to a strong interest in inhibiting RANK ligand (RANKL) signaling to prevent breast cancer. Nevertheless, clinical trial data providing definitive evidence that would allow the adoption of RANKL inhibition in reducing dense breasts and prevent breast cancer are not yet available. We, thereby, propose to (i) perform a randomized clinical trial to quantify the effect of RANKL inhibition with denosumab on mammographic density in high-risk premenopausal women with dense breasts (Primary Aim); (ii) determine the effect of RANKL inhibition on breast tissue RANK, progesterone-regulated pathway gene expression, and related biomarkers associated with breast cancer risk (Secondary Aim). Approach: Study participants will be randomized (1:1) to an intervention (N=116) or placebo arm (N=116). Intervention: The intervention arm will receive two subcutaneous injections of denosumab (60 mg), one at baseline, and a second at 6 months. The placebo arm will receive two subcutaneous placebo injections at baseline, and 6 months. We will use Volpara software to assess mammographic density at baseline, and 12 months. Volpara quantifies volumetric measures of density; volumetric percent density (VPD) allowing us to test differences in change in mammographic density at 12 months among women assigned to intervention vs. placebo. Study population: 232 women undergoing annual screening mammography at the Siteman Cancer Center (SCC), St. Louis, MO. Inclusion criteria: (i) premenopausal; (ii) ?40 years of age; (iii) dense breasts (volumetric percent density ?7.5% on Volpara, equivalent to BI-RADS Category C; (iv) have an increased risk of breast cancer (e.g. positive history of breast cancer in a first-degree relative, non-BRCA susceptibility genes). Target population: Annually, >3,500 premenopausal women with dense breasts aged ?40 years undergo screening mammogram at the SCC, hence, we are confident of reaching our recruitment goals. Impact: Study findings could open up additional therapeutic approaches in primary breast cancer prevention for high-risk premenopausal women with dense breasts, who do not have dominant genetic predisposition.